The invention relates to a process for cooling a strand of metal as it emerges from the mold during continuous casting by applying a coolant directly to the surface of the strand.
Continuous casting with direct cooling is such that the surface of the metal strand emerging from the mold is jetted with coolant immediately below the mold in order to extract heat from the metal. During the so-called start-up phase of the ingot casting, or drop as it is often called, the coolant impinges at first only on the dummy base. The resultant indirect cooling at this stage leads to a moderate solidification of the liquid metal and to a flat walled foot of the ingot. As the dummy base sinks progressively, the coolant strikes the ingot surface directly, which produces a sudden increase in heat extraction from the metal strand. The thermal stresses created as a result of this thermal shock are greater than the yield strength of the ingot, and produce a permanent deformation in the form of a convex curve in the foot of the ingot, and on exceeding the tensile strength of the material leads to cracks in the ingot. In order to produce a continuously cast ingot with a planar base or foot, the strand may not therefore be cooled too strongly during start-up.
A process is known whereby, at least during the start-up phase, the coolant is fed in pulses in order to reduce the cooling intensity.
Also known is a process in which, at least during the start-up, the coolant contains gas introduced under pressure. As the coolant strikes the surface of the ingot, the gas dissolved in the coolant forms an insulating film which reduces heat extraction and therefore the cooling effect.